[llvm-mca] Use a small vector for instructions in the EntryStage.

Use a simple SmallVector to track the lifetime of simulated instructions.
An ordered map was not needed because instructions are already picked in program
order. It is also much faster if we avoid searching for already retired
instructions at the end of every cycle.
The new policy only triggers a "garbage collection" when the number of retired
instructions becomes significantly big when compared with the total size of the
vector.

While working on this, I noticed that instructions were correctly retired, but
their internal state was not updated (i.e. there was no transition from the
EXECUTED state, to the RETIRED state). While this was not a problem for the
views, it prevented the EntryStage from correctly garbage collecting already
retired instructions. That was a bad oversight, and this patch fixes it.

The observed speedup on a debug build of llvm-mca after this patch is ~6%.
On a release build of llvm-mca, the observed speedup is ~%15%.

llvm-svn: 346487

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